The invention is a method for retarding the escape of carbon dioxide gas from a water system. It relates to methods of maintaining low pH levels in moving water systems in which some degree of aeration normally occurs, for example: cooling tower systems, swimming pool filtering systems, farm irrigation systems, and the like.
A prime example of the problems plaguing the prior art is to be found in the operation of a heat exchange system where cooling water containing dissolved solids (hard water) is pumped from the cooling tower basin through heat exchange means and then returned to the cooling tower to be trickled down again to the basin. In the last-named step considerable aeration and evaporation of the water occur in the tower. Due to evaporation the concentration of dissolved mineral solids in the water is increased.
When the concentration of the dissolved solids exceeds its solubility in the aqueous system, scale deposits on the surfaces of the system. For example, calcium carbonate scale builds up on the water side of the heat exchanger tubes when its solubility is exceeded.
The prior art workers learned that lowering the pH of the water in the system increases the solubility of calcium carbonate and other minerals in the water and this principle is used to permit the use of higher concentrations of dissolved minerals. However, an acid must be used to lower pH. (The resulting aqueous system then becomes corrosive, so a corrosion inhibiter such as chromate must be used to prevent damage to the system. (In this respect equations have been developed in the prior art that make it possible to predict whether water has a tendency to be corrosive or to be scale forming. For example, see "Cooling-Water Calculations", by R. G. Kunz, et al, Chemical Engineering, Aug. 1, 1977 and Nalco Reprint Summary No. 20 of an article by John W. Ryznar, Nalco Chemical Company, entitled "A New Index of Determining Amount of Calcium Carbonate Scale Formed by Water".)
The acid of choice in the prior art has been sulfuric acid. It is a cheap and plentiful acid--but dangerous. Because sulfuric acid is a strong acid and dangerous to store and use, attempts to substitute other suitable low cost acids have been made over the years. Carbon dioxide gas has been considered for use as an acid in circulating cooling tower water to control pH. If practical, it would provide a safe, economical alternative to the dangerous sulfuric acid. However, the prior art discounted the use of carbon dioxide because of the unacceptably high loss of carbon dioxide to the atmosphere when the water is pumped over the cooling tower. The unsuitability of carbon dioxide for this reason is noted on page 76, "Principles of Industrial Water Treatment", First Edition 1977, published by Drew Chemical Corporation, Boonton, New Jersey 07005, and page 195, "Handbook of Industrial Water Conditioning", 7th Edition 1976, published by Betz Laboratories, Inc., Trevose, Pa. 19047. As a result, the more dangerous sulfuric acid has continued to be the acid of choice in the prior art.
As can be seen, a felt need exists for a method of retarding the loss of carbon dioxide from circulating water to enhance the utility of carbon dioxide as a source of acid to lower and maintain pH. The invention hereafter described meets that need.